Extrude the bottom edges below the lips (3 times will do) to create the chin. Also see the edges I have just extruded; you do that too, thats going to be the smile wrinkle.

Make two edgeloops between the edge we extruded, and weld the chin end first. Then create one more edge between the two edges (see the picture) and move the new edge close to the upper edge to create a sharp corner there.

Weld the other end to the nostril wing, and tweak the mesh a bit. Also create an edgeloop to the lower-left corner.